Failure diagnosis device for O2 sensor

ABSTRACT

A failure diagnosis device for an O 2  sensor includes an O 2  sensor; a heater driving unit driving a heater heating the O 2  sensor; and a diagnosis unit controlling the heater driving unit heating the heater, and diagnosing O 2  sensor failure after a given amount of time based on an O 2  sensor activation status determination using O 2  sensor outputs. The diagnosis unit includes a heater controlling unit controlling the heater driving unit; a time measuring unit measuring time that electric current is supplied to the heater, and time used in determining sensor abnormality; an activation status determining unit determining air/fuel ratio changes from a lean to rich state, or from a rich to lean state based on the O 2  sensor output; and a failure determining unit determining sensor abnormality based on outputs from the time measuring unit and the activation status determining unit, thereby diagnosing O 2  sensor failure without engine operation.

[0001] This application is based on Application No. 2002-33979, filed in Japan on Feb. 12, 2002, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a failure diagnosis device for an O₂ sensor used in a fuel injection control system of an internal combustion engine.

[0004] 2. Description of the Related Art

[0005] In a fuel injection control system of an internal combustion engine, an O₂ sensor is used as air/fuel ratio detection means for detecting an air/fuel ratio in exhaust gas from an internal combustion engine. In order to determine whether the O₂ sensor is operating normally, a determination is made by taking advantage of the fact that output voltage from the O₂ sensor changes near the theoretical air/fuel ratio. Specifically, by making the engine operate under combustion conditions that would cause the exhaust gas to enter a rich state, it becomes possible to judge that the sensor is operating normally when the output from the O₂ sensor changes from a lean state to the rich state.

[0006] However, in the O₂ sensor failure diagnosis according to the conventional example described above, it is also necessary to have the engine actually running, and it is necessary that the engine run at an air/fuel ratio richer than the theoretical air/fuel ratio. Accordingly, exhaust gas is generated during the internal combustion engine manufacturing process, thus having a negative effect on the environment.

SUMMARY OF THE INVENTION

[0007] The present invention has been made in light of the above-mentioned points, and as object thereof is to provide a failure diagnosis device for an O₂ sensor which is capable of performing the O₂ sensor failure diagnosis without operating the engine.

[0008] With the above object in view, the failure diagnosis device for an O₂ sensor according to the present invention comprises an O₂ sensor for detecting an air/fuel ratio in exhaust gas from an internal combustion engine, a heater driving unit for driving a heater for heating the O₂ sensor, and a diagnosis unit for controlling the heater driving unit for heating the heater, and, after heating the heater for a given amount of time, performing a failure diagnosis regarding the O₂ sensor, based on a determination of an O₂ sensor activation status which is made on the basis of an output from the O₂ sensor.

[0009] Therefore, since the determination of the activation status of the O₂ sensor can be performed without operating the engine, the exhaust gas can be reduced in the manufacturing process of the internal combustion engine provided with the O₂ sensor, thereby obtaining an effect of suppressing the emission of environmentally hazardous substances.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the accompanying drawings:

[0011]FIG. 1 is a block diagram showing a construction of a failure diagnosis device for an O₂ sensor in accordance with an embodiment of the present invention;

[0012]FIG. 2 is a diagram showing an outline of circuitry structure of a diagnosis means in accordance with the embodiment of the present embodiment;

[0013]FIG. 3 is a time chart showing output voltage from the O₂ sensor according to the embodiment of the present invention;

[0014]FIG. 4 is a flow chart showing content of controls performed by a CPU which constitutes a main part of the diagnosis means in accordance with the embodiment of the present embodiment;

[0015]FIG. 5 is a flow chart according to a conventional example shown in comparison to FIG. 4; and

[0016]FIG. 6 is a time chart showing output voltage from an O₂ sensor when a determination of an activation status is performed, in accordance with the conventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIG. 1 is a block diagram showing a construction of a failure diagnosis device for an O₂ sensor in accordance with an embodiment of the present invention. In FIG. 1, reference numeral 1 indicates an O₂ sensor. Reference numeral 2 indicates a diagnosis means which heats a heater by controlling heater drive means discussed below for driving the heater to heat the O₂ sensor 1, and, then, after heating the heater for a given duration of time, performs a failure diagnosis on the O₂ sensor based on a determination of whether the O₂ sensor is active, as determined based on an output from the O₂ sensor 1. The diagnosis means 2 includes an activation status determining means 3 for determining the activation status of the O₂ sensor 1; a heater control means 4 for controlling the heater drive means that is described below; a time measuring means 5 for measuring time that an electric current is supplied to the heater control means 4, and for measuring time that is used in performing the determination of sensor abnormality; and a failure determination means 6 for determining sensor abnormality based on outputs from the time measuring means 5 and from the activation status determining means 3.

[0018] Further, reference numeral 7 indicates a heater drive means for driving the heater for heating the O₂ sensor 1, and reference numeral 8 indicates a failure notification means for notifying about the failure when the diagnosis means 2 determines that a sensor abnormality has occurred. Here, the activation status determining means 3 is configured to determine when the air/fuel ratio has changed from a lean state to a rich state or from the rich state to the lean state, based on an output signal form the O₂ sensor 1. The heater control means 4 controls an electric signal to the heater drive means 7, which is provided to heat the O₂ sensor 1 to a temperature which is needed for activation of the O₂ sensor 1.

[0019] Further, when the failure determination means 6 determines that the failure has occurred, the failure notification means 8 makes a failure notification. The failure notification means 8 may be a lamp or buzzer or the like for making the notification directly, and may also include sending a failure signal to another display apparatus or failure diagnosis device by using communications.

[0020]FIG. 2 is a diagram showing an outline of a circuitry structure of the diagnosis means 2 according to the present embodiment. As shown in FIG. 2, the diagnosis means 2 includes a CPU 2 a. An input stage of the CPU 2 a is provided with an I/F circuit 2 b, and between the I/F circuit 2 b and the O₂ sensor 1 there is provided a pull-up resistor 2 c for performing a pull-up at the same power source voltage as the CPU 2 a. A divided voltage value obtained through the pull-up resistor 2 c and an internal impedance exhibiting a negative coefficient of the O₂ sensor 1, is inputted into the CPU 2 a via the I/F circuit 2 b as an output value from the O₂ sensor 1. For the failure diagnosis and to drive a heater 9 for the O₂ sensor 1, the CPU 2 a outputs a heater control signal to the heater drive means 7, thereby heating the O₂ sensor 1. Further, in a case where the CPU 2 a judges that a failure has occurred, the CPU 2 a outputs, to a transistor 2 d and thus to a failure notification means 8, an output signal indicating occurrence of a failure. In accordance with the present invention, the failure notification means 8 is a lamp. Further, the heater drive means 7 is constituted by two transistors and a resistor.

[0021]FIG. 3 is a time chart of the voltage outputted from the O₂ sensor 1 in accordance with the present embodiment. Generally, the voltage outputted from the O₂ sensor 1 is about 0.2 V when the air/fuel ratio is on the lean side, and about 1 V of electromotive force is generated when on the rich side. The internal impedance of the O₂ sensor 1 exhibits a negative coefficient with respect to the temperature. At the time when the power source is turned on, the impedance is great. Therefore, the voltage divided through the pull-up resistor 2 c substantially corresponds to the pull-up voltage. Then, as the O₂ sensor 1 is heated and activated by means of the heater 9, the internal impedance of the O₂ sensor 1 drops. As a result, the electromotive force generated by the O₂ sensor 1 itself is outputted to a terminal.

[0022]FIG. 6 is a time chart showing voltage outputted from an O₂ sensor when a determination of an activation status is made in accordance with a conventional example. In this example, the power source is turned on, and, then, after enough time has elapsed for the O₂ sensor to be sufficiently heated by the heater to thus operate normally, it is then necessary to start the engine and run the engine in such a way that the air/fuel ratio enters the rich side. In the case where the sensor operation is normal, the voltage outputted from the sensor indicates that the air/fuel ratio is on the rich side, and, therefore, it is determined that the sensor operation is normal.

[0023]FIG. 4 is a flow chart showing content of controls performed by the CPU 2 a, which constitutes a main part of the diagnosis means 2, in accordance with the present embodiment. At step S1, the CPU 2 a drives the heater 9 which heats the O₂ sensor 1. At step S2, the CPU 2 a waits for a duration of time necessary to heat the O₂ sensor 1. At step S3, the CPU 2 a judges whether the sensor output indicates the lean state. Then, in the case where the lean state is detected, the control advances to step S4. In the case where the rich state is detected, the control advances to step S5.

[0024] At step S4, a flag is set to indicate that the O₂ sensor 1 is activated, and at step S5 a determination of a time duration is performed. If the set duration of time has not elapsed, the control returns to step S3. If the set duration of time has elapsed the control advances to step S6, and a flag indicating abnormality is set.

[0025] The flow chart according to the embodiment shown in FIG. 4 is different from the flow chart according to the conventional example shown in FIG. 5 in that step S7 of driving the engine to make the determination of the activation status is eliminated.

[0026] Therefore, since the above-mentioned embodiment enables determination of an O₂ sensor activation status without running the engine, it becomes possible to reduce exhaust gas in the manufacturing process of an internal combustion engine provided with the O₂ sensor. Thus, emission of environmentally hazardous substances can be suppressed.

[0027] Further, the change in the O₂ sensor's internal impedance when the O₂ sensor is activated by itself can be learned from the sensor signal obtained through the voltage division through the pull-up resistor that is added to the detection circuit in the control device. As a result, it becomes possible to determine whether the O₂ sensor is operating normally without having to operate the engine.

[0028] As described above, in accordance with the present invention, since the determination of the activation status of the O₂ sensor can be performed without operating the engine, the exhaust gas can be reduced in the manufacturing process of the internal combustion engine provided with the O₂ sensor, thereby obtaining an effect of suppressing the emission of environmentally hazardous substances. 

What is claimed is:
 1. A failure diagnosis device for an O₂ sensor, comprising: an O₂ sensor for detecting an air/fuel ratio in exhaust gas of an internal combustion engine; a heater drive means for driving a heater for heating the O₂ sensor; and a diagnosis means for controlling the heater drive means for heating the heater, and, after heating the heater for a given amount of time, performing a failure diagnosis regarding the O₂ sensor, based on a determination of an O₂ sensor activation status which is made based on an output from the O₂ sensor.
 2. The failure diagnosis device for an O₂ sensor according to claim 1, wherein the diagnosis means performs the determination of an activation status by determining that the sensor is active when the air/fuel ratio is determined to be in a lean state based on the output from the O₂ sensor, and by determining that the sensor operation is abnormal in a case where a rich state of the air/fuel ratio continues for a given duration of time.
 3. The failure diagnosis device for an O₂ sensor according to claim 1, wherein the diagnosis means comprises: a heater control means for controlling the heater drive means; a time measuring means for measuring a time duration that an electric current is supplied to the heater control means, and for measuring time that is used in performing the determination of the sensor abnormality; an activation status determining means for determining a change in the air/fuel ratio from the lean state to the rich state, or from the rich state to the lean state, based on the output from the O₂ sensor; and a failure determining means for determining the sensor abnormality based on outputs from the time measuring means and from the activation status determining means.
 4. The failure diagnosis device for an O₂ sensor according to claim 1, further comprising a failure notification means for making a notification of failure in the case where the diagnosis means determines that the sensor operation is abnormal.
 5. The failure diagnosis device for an O₂ sensor according to claim 1, wherein: the diagnosis means is composed of a CPU; an input stage of the CPU is provided with an I/F circuit, and between the I/F circuit and the O₂ sensor there is provided a pull-up resistor for performing a pull-up at the same power source voltage as the CPU; and a divided voltage value which is obtained through the pull-up resistor and an internal impedance exhibiting a negative coefficient with regard to the temperature of the O₂ sensor, is inputted into the CPU via the I/F circuit as the output from the O₂ sensor. 